We propose to elucidate the molecular processes coupling the binding of oytocin to receptor sites and the contraction of target cells. Myoepithelial cells from the mammary gland of the lactating rat will be isolated and purified. The effect of oxytocin on 45Ca exchange, ATPase actvity and protein phosphorylation will be determined with intact cells and purified plasma membranes. The concentration of oxytocin giving half-maximal stimulation will be compared to the Km of oxytocin-induced contractions of the isolated cells. The relative importance of extra-cellular and intracellular stores of Ca2 ion in oxytocin action will be studied. The subcellular sites of phosphorylation will be studied and phosphorylated proteins will be resolved by SDS-gel electrophoresis. The mechanisms of ATP inhibition of oxytocin binding and of oxytocin antagonist action will be studied kinetically. We will try to solubilize and purify oxytocin receptor from myoepithelial cells by affinity chromatography, and to prepare specific antibodies against receptors from rat uterus. Oxytocin target cells in endometrium will be localized immunocytochemically with antibody to oxytocin. The ultrastructural location of receptors in myoepithelial and myometrial cells also will be determined by immunocytochemistry. We will test the hypothesis that labor is induced in the rat by the appearance of receptors for oxytocin. The effects of PGF2alpha and prostaglandin inhibitors on oxytocin binding to the pregnant rat uterus will be studied. Blood levels of oxytocin in pregnant rats will be measured by immunoassay.